Thom,I will break it down and fully answer your questions. It will take me a bit to get the full response, but I will

Sharkcutup,We bought this machine based on what it was advertised to do.If a $5000.00+ machine cannot properly carve a pocket, why on earth would anyone spend one red cent on it, when there are many other options out there that can?We went through forums and dug through pros/cons of buying this machine. Based on all of the rave reviews, we decided to make the investment.

Hopefully, it is not the machine, but something small hidden in the details, and fixable.Next Wave Automation is currently working with us to resolve the issue as well.

I have actually considered doing what Thom suggested and breaking down the design. However, at this point, I am not convinced it is a design issue. That said, I will certainly be following Thom's advice on troubleshooting.

xoneeleven wrote:Thom,I have actually considered doing what Thom suggested and breaking down the design. However, at this point, I am not convinced it is a design issue. That said, I will certainly be following Thom's advice on troubleshooting.

The breaking down of the design, that I requested, was to get rid of any confounding issues that might either mask the issue, inadvertently cause the issue, or just make the troubleshooting process needlessly longer.

I agree this is not a design issue. Nor a post-processor issue. But, in some cases, it also might be unavoidable without the application of $$$.00.

If I'm correct (admittedly a big IF), the root cause is that the stepper motors don't actually work at the same 0.001" resolution (or the equivalent in mm), because they are on a 1/4" per turn lead screw (two threads), but the motors only have 220 steps. That means that each full step of the motor is actually 0.00114". When the virtual Z is used to create a flat pocket, and the virtual-z offsets are very small, the z-axis movement demonstrates the equivalent of aliasing. If you can step-jog incrementally by 0.001", you'll hear a distinctly different sound as you jog through every about 7th increment (1/7 = 0.1428...).

I have previously seen what I believe might be the same thing. In my case, they were completely benign, but visibly present in a 0.002" deep engraving that was mapped onto the slightly sloped surface of the part. The below image is about 1/4" top-to-bottom.

0.002" deep engraving into a glass-bead blasted aluminum surface

Manual touch-up of those parts is not possible, especially not when I make them in production lots of 50 pieces.

In my case, I didn't use the virtual Z, but instead mapped an engraving path (again, restricted to 0.002" deep) onto a slighly inclined model of the actual deformed part stock (0.010" deformation across 1.5"). Yeah, it turns out that 90-degree aluminum angle isn't exactly 90-degrees when you're talking thousandths accuracy. The issue of motor resolution versus system resolution is detectable as a regularly-spaced just-slightly-off depth engraving that is not evident in the GCode, even when calculated to six decimal places. And yes, you guessed it: slightly off BELOW the expected surface.

If I had to guess, you're using a non-radiused (aka sharp-tipped) endmill, and your spindle/router is just slightly out of "tram". When the virtual-Z is calculating the needed position, it's adding very tiny amounts into the movement. Across wide areas of only very small height deviation, those aliasing effects can show up, and when combined with the out-of-tram spindle, make that visible. To help alleviate this, use radiused endmills (e.g., 0.030 radius on a 1/4" bit) so the transitions at the edge of the cut will be smoother, and ditch virtual-Z.

To put it plainly, wood warps as a cylinder. A cylinder cannot be accurately modeled with a pyramid, which is what virtual-Z tries to do.

To my mind, when working with reasonably flat wood, virtual-Z is more of a pain than it's worth. And when you're clearing out a big area, it's useless. The idea behind virtual-Z was that it could more-accurately place shallow features onto a surface where the surface deformation is significant WRT the intended features. Unfortunately, it requires a minimum of 9 points to even begin to model a cylinder. I'm guessing they decided their customers wouldn't put up with that many touch-offs. Not to mention that flat touch-off plate makes it difficult to even accurately capture a concave surface.

V-carved paths are especially vulnerable to the effects of different surface heights, since the change in apparent path width is very perceptible to the human mind. It's kind of like how easy it is for us to detect when circles aren't truly concentric. But, when you're trying to create an at-depth, flat surface, you don't care AT ALL what the old surface deformation was, because you're cutting it away.

Therefore, when I'm clearing any flat area down from the surface, I would never use virtual-Z. In fact, I never use virtual Z at all, because the implementation does not properly accommodate the material deformation it was intended to overcome. But, for me, it's really more because I'm doing aluminum smaller parts. If the material stock will be "normally" deformed, and I need a shallow feature carved in, I'll build a model in Fusion that represents that deformed stock, and then map the toolpath onto the deformed part's model, then cut that. And, if there are minor deviations where the model isn't exact, I'll use my GCodeShim tool to nudge a section of the toolpath in the appropriate direction. But, none of those techniques gets away from the underlying aliasing of vertical positioning: it's still there.

The reason, as I'm sure you're guessing, that you don't see any problems in the GCode, is that virtual-Z height offsets are added AFTER the GCode is produced.

Now for the good/bad news. This issue will be present on all machines whose native resolution is not actually the 0.001" the software systems believe. And by that I mean pretty much ALL machines that use a) 200/220-step motors, and b) that cannot micro-step with sufficient position-holding torque for the Z-axis. The shark, and potentially all units in the <$20K range will exhibit this behavior. The only real way to avoid it is to get a machine with true z-axis resolution well under 0.001". That can also be fixed with a 10x zero-backlash reducing gear on the Z-axis. It depends on how important this is to you, just how much you're willing to spend getting rid of it.

But for you, right now, I believe the simplest way to get rid of those, then, is to stop using virtual-Z All those questions? Those would allow us to positively verify that is exactly what is happening at that moment in the toolpath.

Seem plausible? And yes, I'm okay with being completely wrong, if you're not actually using Vitual-Z. . If I'm not proven wrong every day, then I'm not trying hard enough!

Regards,

Thom

Last edited by Rando on Mon May 15, 2017 10:28 am, edited 8 times in total.

Sharkcutup,We bought this machine based on what it was advertised to do.If a $5000.00+ machine cannot properly carve a pocket, why on earth would anyone spend one red cent on it, when there are many other options out there that can?We went through forums and dug through pros/cons of buying this machine. Based on all of the rave reviews, we decided to make the investment.

Hopefully, it is not the machine, but something small hidden in the details, and fixable.Next Wave Automation is currently working with us to resolve the issue as well.

I have actually considered doing what Thom suggested and breaking down the design. However, at this point, I am not convinced it is a design issue. That said, I will certainly be following Thom's advice on troubleshooting.

No disrespect intended to you but just an overall opinion about the machine was all!!! I still say there is too much so called "plastic" in the machine.

With the above said: I am in the process --- one step at a time replacing my CNC so called "plastic" with aluminum!

GREAT!!! Glad to see you have been able to get Next Wave into the mix (they are usually difficult/tough (and non-existent on weekends) to get a hold of)!

Thom is very knowledgeable! If anyone can solve your mystery he most probably can!!!

Again Good Luck with your troubleshooting/problem solving!!! Also want to say that your project is looking great thus far! The brain storming is certainly paying off!!!

Sharkcutup,We bought this machine based on what it was advertised to do.If a $5000.00+ machine cannot properly carve a pocket, why on earth would anyone spend one red cent on it, when there are many other options out there that can?We went through forums and dug through pros/cons of buying this machine. Based on all of the rave reviews, we decided to make the investment.

Hopefully, it is not the machine, but something small hidden in the details, and fixable.Next Wave Automation is currently working with us to resolve the issue as well.

I have actually considered doing what Thom suggested and breaking down the design. However, at this point, I am not convinced it is a design issue. That said, I will certainly be following Thom's advice on troubleshooting.

No disrespect intended to you but just an overall opinion about the machine was all!!! I still say there is too much so called "plastic" in the machine.

With the above said: I am in the process --- one step at a time replacing my CNC so called "plastic" with aluminum!

GREAT!!! Glad to see you have been able to get Next Wave into the mix (they are usually difficult/tough (and non-existent on weekends) to get a hold of)!

Thom is very knowledgeable! If anyone can solve your mystery he most probably can!!!

Again Good Luck with your troubleshooting/problem solving!!! Also want to say that your project is looking great thus far! The brain storming is certainly paying off!!!

Have a GREAT DAY!!!

Be SAFE around those AWESOME machines!!!

Sharkcutup

I did not take offense to anything you said, whatsoever. I am irritated at the machine, and that is all.Forgive me if I came off as abrasive.

Thom,We are not using virtual Z whatsoever.All we have been using is the touch plate at the start of the job, and when changing bits.

Your project looks nice, and is much more sophisticated than what I am currently capable of.

Has anyone here that uses a CNC Shark with the Shark Control software v2 ever successfully cut a large pocket without seeing similar grooves?If anyone has been able to do this sort of pocketing, then I am really hoping that it is not a machine design issue.Even with the plastic parts on the CNC Shark, I have watched videos where people cut pockets and do not have this issue.

That said, tramming might play some part. I may start looking closer at that angle.

Please firgive the sloppyness in my replies. I am working outside and comin gin during breaks to respond.

As you work in finer structures, it will crop up again, so keep it in mind. I see it more and more now that I recognize it for what it is.

Then, I guess the last few things I'd check that both ends of the z-axis motor coupling are quite and truly locked in place, with the setscrew against the D-flat and not allowing any back-and-forth backlash. And, potentially the z-axis backlash nuts, but you're on a relatively new machine. I'll assume you haven't had a habit of repeatedly running the z-axis to the end of its travel.

Yeah, I guess I'd try and stop the thing right when it's making a cut and see if the reported z-axis position is what it is supposed to be according to the GCode, but I suspect it will claim it's own perfection .

Was your design, by any chance, started from a photograph that you did a darkness-to-height translation to get the vectors? Nah, that would show up in the GCode.

Yeah, at this point I'm going to say, like the ~0.018" touch-off height issue, this one is just "in there somewhere", and probably is the result of some rounding error in somebody's calculation. Debugging motion control issues like this, without having the source code of that controller, much less being able to affect a change to it, makes the trail essentially end at the USB port or memory stick.

That touch-off issue is this: if you use a conductive piece of stock in a vise that's well connected to the bed, you can touch off directly on the material. And, when you home to 0,0,0 it goes precisely there. But, if you run GCode that says move the bit to Z0, I get a divot 0.018 to 0.020" deep. That's why I always touch off with an 0.020" thick bit of aluminum sheet. It protects the stock from tiny marks made int the touch-off process as well, a big concern when the block has already been mirror-polished.

I used only the circle around the large HEbrew letters to create a single tool path to cut at a slightly deeper debth of 0.22in (previous pocket cut depth was 0.2in).

The job is complete, and there are no visible recessed lines, yet. The pocket looks exactly as a pocket should look.Could then, the issue be related to one of the following? - Limitations in the steppers that Thom talked aboutor- Bug with the CNC Shark Control Software v2, which cannot carve reliably at 0.2in depthor- CNC Shark Control Software cannot reliably carve a VCarvePro set of toolpaths where a pocket is set as a toolpath with a clearance path and a finish path.

These are the only things I can think of from the experience so far. (Although I need to also look into some of Thom's ideas)

We opened a ticket with Next Wave Automation a few weeks ago, and actually talked to one of their long-timers.

He said the issue is likely one of two things, or both:

1: The CNC Shark Dust Boot (yes the one from Rockler at roughly $100.00 after shipping)... They said the dust boot brush is too stiff and they have seen scenarios that were fixed by removing the dust boot. (frankly, I believe it is a weight issue)

2: If you have dust collection, and a standard 4 inch hose attached, it could produce too much weight on the downward Z.

So, we began a test on a pine board (our testing materials are getting cheaper and cheaper). We used the same design as before, but scaled it down to 12"x12".We removed the dust boot, but kept the hose connected & ran the clearance path job. Halfway through, we noticed the same grooves appearing. However, they were not as pronounced as all of the previous runs. This is a good start, but still unacceptable results.

Next, we paused the job and removed the vacuum hose. Now both the dust boot and hose are removed. The results were much better. Although there were some faint signs of two grooves, they were at an acceptable level. (very faint and easy to remove with sanding).

My guess is that the motors are not strong enough for the size of the machine. Why in the world would anyone buy a CNC that you cannot put a dust boot or vacuum on it????????

I am dying to find out what you all have to say about this, if this topic is even chiming you!